1. Field of the Invention
The present invention provides a novel amino acid derivative, and a salt for thereof. The present invention further provides a sweetening agent and a food or beverage, which contains the novel amino acid derivative of the present invention to increase the sweetness of the same.
2. Discussion of the Background
In recent years, there has been an increased incidence of problems resulting from excessive ingestion of sugar, for example obesity and various types of diseases accompanied thereby. Therefore, development of a low-calorie sweetening agent to serve as a sugar substitute is in high demand. In addition to strength of the sweetness, several additional characteristics and requirements should also be satisfied by the sugar-substitute including: low-calorie, high safety (i.e., little or no side effects), high stability against heat or acid, excellent sweetness quality, and low cost.
Currently, several types of sweetening agents have been used or proposed. For example, aspartame has gained notoriety as a widely used sweetening agent, due to its potent sweetness strength and quality as well as its ease for industrial manufacture on a large scale and its excellent safety. Furthermore, studies on aspartame derivatives have also been extensively conducted. In addition thereto, sweetening materials having various characteristics have been proposed as a sweetening agent and investigations toward the practical use thereof have been conducted. Additional sweetening agents that are currently used include naturally occurring thaumatin, glycyrrhizin, stevioside and the like which are derived from plants and can be collected on a large scale.
Although not yet being used practically as a sweetening agent, monatin has been known as a natural sweetening material. Monatin is a naturally occurring amino acid derivative isolated from root bark of Schlerochiton ilicifolius which is a self-sown plant in the Northwestern Transvaal region of South Africa, and R. Vleggaar et al. (J. Chem. Soc. Perkin Trans., 3095-3098, (1992)), reported its structure as being (2S,4S)-2-amino-4-carboxy-4-hydroxy-5-(3-indolyl)pentanoic acid ((2S,4S)-4-hydroxy-4-(3-indolylmethyl)-glutamic acid (see, Chemical formula (3)). Additionally, according to Vleggar et al. the degree of sweetness of the (2S,4S) isomer referred to as being derived from this natural plant has been reported to be 800 times, or maybe 1400 times of sucrose. However, a study of the relationship between the chemical structure of monatin and the appearance of the sweetness has not been performed. Thus, it has not been elucidated as to which functional group is required for the sweetness of monatin. Accordingly, no attempt has been conducted to obtain a novel sweetening material by using monatin as a lead compound and modifying the chemical structure.

Although some processes for the synthesis of monatin have been reported, a suitable industrial process has not been reported to date. Examples of synthesis of monatin can be found in South Africa (ZA) Patent Application No. 87/4288, C. W. Holzapfel et al., Synthetic Communications, 24 (22), 3197-3211 (1994), U.S. Pat. No. 5,994,559, and K, Nakamura et al., Organic Letters, 2, 2967-2970 (2000). Therefore, a derivative having a sweetness strength equivalent to or greater than that of monatin, which can be more readily produced than monatin, is desired and would have more feasible practicability as a sweetening agent.
The aforementioned South Africa (ZA) Patent Application No. 87/4288 discloses that monatin produces intramolecular lactone or lactam through cyclodehydration under a specified condition. It is not clear whether these cyclized products have sweetness or not. However, assuming that these cyclized products do not have sweetness, it is believed that derivatives of monatin that do not yield or are difficult to yield such products are more preferred sweetening materials in terms of stability when used as a sweetening agent.
On the other hand, D-6-chlorotryptophan has been known as a sweetening material, which has an amino acid structure similarly to monatin in terms of the chemical structure. Among the analogs thereof, known amino acids exhibiting sweetness are limited only to compounds having a substituent at an indole ring of tryptophan (see, JP-A-S48-16624).
Therefore, elucidation of a chemical structure required to impart sweetness of monatin is desired, as well as determining the chemical structure of a sweetening agent having superior properties required for a sweetening agent than conventional sweetening materials.